This is avr-libc. It is a C library implementation for use with GNU GCC and GNU binutils for development of programs for Atmel's AVR microcontrollers. The current branch (1.3), requires the following versions of gcc and binutils: Tool Minimal Version Preferred Version =========== =============== ================= binutils 2.13 Latest gcc 3.3 Latest 3.4.x For details on installing and using AVR Libc, see the users manual in the doc/api directory or online at: http://savannah.nongnu.org/projects/avr-libc/